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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
</CENTER>
<HR>
<H3>compute vacf command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>compute ID group-ID vacf
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "compute.html">compute</A> command
<LI>vacf = style name of this compute command
</UL>
<P><B>Examples:</B>
</P>
<PRE>compute 1 all vacf
compute 1 upper vacf
</PRE>
<P><B>Description:</B>
</P>
<P>Define a computation that calculates the velocity auto-correlation
function (VACF), averaged over a group of atoms. Each atom's
contribution to the VACF is its current velocity vector dotted into
its initial velocity vector at the time the compute was specified.
</P>
<P>A vector of four quantites is calculated by this compute. The first 3
elements of the vector are vx * vx0 (and similarly for the y and z
components), summed and averaged over atoms in the group. Vx is the
current x-component of velocity for the atom, vx0 is the initial
x-component of velocity for the atom. The 4th element of the vector
is the total VACF, i.e. (vx*vx0 + vy*vy0 + vz*vz0), summed and
averaged over atoms in the group.
</P>
<P>The integral of the VACF versus time is proportional to the diffusion
coefficient of the diffusing atoms. This can be computed in the
following manner, using the <A HREF = "variable.html">variable trap()</A> function:
</P>
<PRE>compute 2 all vacf
fix 5 all vector 1 c_2[4]
variable diff equal dt*trap(f_5)
thermo_style custom step v_diff
</PRE>
<P>IMPORTANT NOTE: If you want the quantities calculated by this compute
to be continuous when running from a <A HREF = "read_restart.html">restart file</A>,
then you should use the same ID for this compute, as in the original
run. This is so that the fix this compute creates to store per-atom
quantities will also have the same ID, and thus be initialized
correctly with time=0 atom velocities from the restart file.
</P>
<P><B>Output info:</B>
</P>
<P>This compute calculates a global vector of length 4, which can be
accessed by indices 1-4 by any command that uses global vector values
from a compute as input. See <A HREF = "Section_howto.html#howto_15">this
section</A> for an overview of LAMMPS output
options.
</P>
<P>The vector values are "intensive". The vector values will be in
velocity^2 <A HREF = "units.html">units</A>.
</P>
<P><B>Restrictions:</B> none
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "compute_msd.html">compute msd</A>
</P>
<P><B>Default:</B> none
</P>
</HTML>
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